Method of fabricating lightweight electrodes

ABSTRACT

An electrode with a noble metal loading of no greater than 1 mg./cm.2 is prepared by depositing an alloy catalyst on a finely divided support, mixing the supported alloy catalyst and a hydrophobic polymer, suspending the mixture in a wetting agent water solution, applying the suspension onto a substrate, pressing the coated substrate, drying the coated pressed substrate and sintering the coated, pressed, dried substrate.

United States Patent 1 Breault et al.

1 1 Oct. 2, 1973 22 Filed: June8, 1970 21 Appl.No.:44,498

[52] U.S. Cl. 136/121 [51] Int. Cl. H0lm 13/02 [58] Field of Search136/120 FC, 86 D,

[56] References Cited UNITED STATES PATENTS 3,444,004 5/1969 Smith136/86 D 3,549,423 1 12/1970 Grubb et al 136/120 FC 3,238,068 3/1966Hipp 136/120 FC 3,440,107 4/1969 Barber 136/120 FC PrimaryExaminer-Winston A. Douglas Assistant Examiner-M. J. AndrewsAttorney-Laurence A. Savage [57] ABSTRACT An electrode with a noblemetal loading of no greater than 1 mgjcm? is prepared by depositing analloy catalyst on a finely divided support, mixing the supported alloycatalyst and a hydrophobic polymer, suspending the mixture in a wettingagent water solution, applying the suspension onto a substrate, pressingthe coated substrate, drying the coated pressed substrate and sinteringthe coated, pressed, dried substrate.

7 Claims, 1 Drawing Figure PFC JS/A 7 19/5 (O/972. 6 JUEJTf/Q/" METHODOF FABRICATING LIGHTWEIGHT ELECTRODES BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to the fabrication of fuelcell electrodes and particularly to lightweightelectrodes for use infuel cells.

2. Background of the Invention In fuel cells utilizing electrodes havinga noble metal catalyst, it is desirable to use as little noble metalaspossible because of the extremely high cost of the noble metals, whilestill providing an electrode which operates as efficiently as electrodeshaving a high loading of noble metal. Electrodes are known in the artwhich have a noble metal loading of mg./cm or greater, and electrodeshave been proposed having a lower metal noble loading; However, thoseelectrodes known heretofore which have a lower loading have fallen shortof desired performance and efficiency and do not operate as efficientlyas the more highly loaded electrodes. The cost of producing an electrodeaccording to our invention is approximately l/ of the cost of acommercially available electrode having, for example, a noble metalloading of IO mgJcm, (5 mg./cm. of platinum and 5 mg./cm. of rhodium).

SUMMARY OF THE INVENTION It is the object of this invention to providea. lightweight fuel cell electrode which contains less than 1 mg./cm. ofnoble metalwith performance better than, or equivalent to, knownelectrodes having a noble metal content of 10 mg./cm. or greater.

It has been found that the foregoing object may be readily attainedthrough the use of the lightweight electrode fabrication techniquedisclosed herein. In accordance with our invention a method of forming alightweight electrode havinga noble metal content of less than 1 mg./cm.comprises mixing a supported alloy catalyst and a hydrophobic polymerand'suspending the admixture in a wetting agent/water solution; applyingthe suspension onto a substrate in a thin concentrated layer so that thenoble metal content is no greater than 1 mg./crn.. The substrate may be,for example, either a screen having a thin layer of metal foil, waxpaper or a thin sheet of polytetrafluoreth-ylene pressed into it to fillapproximately one half the void volume of the screen, or the substratemay be a hydrophobic carbon paper; pressing the coated substrate; dryingthe coated, pressed substrate to remove the wetting agent; and sinteringthe coated, pressed, dried substrate to bind the polymer particlestogether, make them more hydrophobic and to increase the structuralintegrity of the electrode.

Other objects, features, and advantages of the present invention willbecome more apparent in light of the following detailed description of apreferred embodiment thereof, as illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a block diagram of amethod of fabricating. a lightweight fuel cell electrode in accordancewith our invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A method of forming alightweight fuel cell electrode containing less than 1 mg./cm. of noblemetal is illustrated in the FIGURE. In accordance with this invention analloycatalyst, which may consist of weight percent platinum and 30weight percent ruthenium as a finely divided powder, is deposited on afinely divided support, such as graphite powder, in the proportions ofIO weight percent alloy catalyst and weight percent finely dividedsupport. The supported alloy catalyst is thenmixed with a hydrophobicpolymer such as fluorinated hydrocarbon (for examplepolytetrafluoroethylene or copolomers of polytetrafluoroethylene andpolyhexafluoropropylene) as a powder or as colloidal particles insuspension in a ratio of 4 parts supported alloy catalyst to one parthydrophobic polymer (by weight), and suspending the admixture in awetting agent/water solution, such as an alkylaryl polyetheralcohol/watersolution in a ratio of one gram of alloy catalyst per 5milliliters of solution. A satisfactory alkylaryl polyether alcohol isTriton. X-IOO (manufactured by Rohm & Haas Co.). The wetting agent/watersolution is a very necessary ingredient because its role is to dispersethe hydrophobic polymer uniformly over the catalyst. The suspension isthen applied to the substrate by any of the known means, such asspraying, for example, and the coated substrate may then, if desired, beair dried at a temperature in the range of to l80F to remove the water.The coated screen. is then pressed at a pressure in the range of 400 to3,500 psi depending on the type of substrate used 3,500 1,000 to psi fora screen substrate and 400 to 2,000 psi for a carbon paper substrate),and the coated, pressed substrate is then dried at a temperature in therange of 250 to 350F to remove the wetting agent. The coated, pressed,dried substrate is then sintered at. a temperature in the range of 570to 620F.

A particular screen which is found to be desirable is a 100 meshgold-plated tantalum screen made with a 3 mil wire size. When a screenis used as a substrate, a l .5 mil sheet of aluminum foil is pressedinto the screen at a pressure of about 10,000 psi to fill approximatelyone half the void volume of the screen. This allows the catalyst to beconcentrated in a thin layer on the substrate. Insteadof the aluminumfoil, waxed paper or a thin film of polytetrafluoroethylene (Teflon,forexample) may be used. The aluminum foil or Teflon film can be removedbefore or after pressing by mechanically strippingit off, or can be lefton the screen until after sintering, when the aluminum foil could bedissolved off in potassium hydroxide or phosphoric acid (if Teflon filmis used it can be mechanically stripped off, even after sintering); ifwaxed paper were used instead of the aluminum; foil, it should beremoved prior to sintering.

If it is desired to use a hydrophobic carbon paper as the substrate, itis prepared as follows:

A 70 percent porous carbon paper with a mean pore size of about 70microns is dipped in a 60 weight percent polytetrafluoroethylenedispersion (TFE-30 manufactured by the DuPont Company is acceptable) toacquire a loading of about 20 mgJcm. of the polytetrafluoroethylene.

Although the preferred process stepshave been enumerated, certaincomposition, temperature and pressure modifications may be made withoutsacrificing the product characteristics. For example, any of the impurehydrogen catalysts known to the art can be used. These consist of alloysof platinum with metals such as ruthenium, rhodium, iridium, and nickel.While a composition of 70 weight percent of platinum and 30 weightpercent of ruthenium is preferred, an acceptable range is 50 to 80weight percent of platinum and 20 to 50 weight percent of the othermetal. Finely divided supports other than graphite, such as carbon orboron carbide can also be used. While a composition of weight percentalloy catalyst and 90 weight percent finely divided support ispreferred, a range of 10 to 35 weight percent alloy catalyst and 65 to90 weight percent of support produces an acceptable electrode. Thepreferred ratio of supported alloy catalyst (platinum'rnetal-support) tohydrophobic polymer is 4 parts supported alloy catalyst 1 parthydrophobic polymer (by weight), although 7 parts supported alloycatalyst 3 parts hydrophobic polymer or 9 parts supported alloy catalyst1 part hydrophobic polymer produce acceptable results. The wettingagent/water solution preferrably contains 2.5 to 5.0 volume percentwetting agent, although a composition in the range of 0.2 to 5.0 volumepercent wetting agent produces satisfactory results. The preferredproportion of alloy catalyst (platinum-metal alloy) to suspension is 1gm. catalyst to 5 milliliters of suspension when graphite is used as acatalyst support, but the proportions may go to 1 gm. of catalyst tomilliliters ofsuspension when carbon is used as the support, or 1 gm.catalyst to 4 milliliters when boron carbide is used as the catalystsupport. The pressing step which occurs after air drying may be carriedout at pressures between 1,000 and 3,500 psi when a screen is used andbetween 400 to 2,000 psi when hydrophobic carbon paper is used. Thedrying step may be carried out between 250 and 350F, although 300F ispreferred, and drying time is shortened by drying in a vacuum. Finallythe sintering temperature may be in the range of 570 to 620F, while 590For 620F is preferred.

In order to demonstrate the advantage of the present invention, astructure fabricated in accordance with the described method is comparedto a structure fabricated from a technique known in the art which has anoble metal content of about 10 mg./cm.

EXAMPLE An electrode according to our invention was prepared as follows:A 1.5 mil thick sheet of aluminum foil was pressed into a 100 meshgold-plated tantalum screen with a 3 mil wire size at a pressure of10,000 psi to fill approximately one-half the void volume of the screen.A suspension was prepared by mixing a catalyst consisting of 8 weightpercent of platinum, 2 weight percent of ruthenium and 90 weight percentof boron carbide with Teflon- (manufactured by the DuPont Company) in aratio of 4 parts supported alloy catalyst to 1 part Teflon-3O (byweight); the catalyst-Teflon ad mixture was then suspended in a 2.5volume percent Triton X-lOO/water solution in a ratio of 1 gm. of alloycatalyst per 4 milliliters of solution. The suspension of catalyst andTeflon was then sprayed onto the screen so that the noble metal loadingwas 0.62 mg./cm. The sprayed screen was air dried at 120F to remove thewater. The aluminum foil was then stripped off the eletrode, which wasthen pressed at 1,300 psi, and then vacuum dried for 16 hours at 250F toremove the Triton X-lOO wetting agent. The electrode was then sinteredat 590F for five minutes.

An electrode made by the above-described process was tested as an anodeat 300F in 96 weight percent phosphoric acid. The anode contained 0.62mg./cm. of noble metal (platinum-ruthenium alloy), and typical resultsfor the electrode are tabulated in Table 1.

Typical results for a commercially available electrode containing 10mg./cm. of noble metal (5 mg./cm. platinum 5 mg./cm. rhodium) tested asan anode in 96 weight percent phosphoric acid at 300F are shown in Table11.

TABLE II ANODE POLARIZATION (millivolts) FUEL I00 ASF 200 ASF 300 ASFHYDROGEN 32 56 77 IMPURE HYDROGEN 48 88 127 H 1.3% CO- 18.7% CO,)

A comparison of Table I and Table II demonstrates that the processdescribed (and the electrode fabricated thereby) achieves a lower anodepolarization voltage (in millivolts) for both types of fuels and for allthree current densities in amps per square foot than a commerciallyavailable electrode having a noble metal content of 10 mg./cm. exceptfor one case in which the voltages are essentially equal withinmeasurement tolerances.

Although the invention has been shown and described with respect to apreferred embodiment it should be understood by those skilled in the artthat various changes and omissions in the form and detail thereof may bemade therein without departing from the spirit and scope of theinvention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A method of fabricating a lightweight fuel cell electrode having anoble metal content no greater than 1 mg./cm. comprising the steps of:

preparing an electrically conductive substrate by dipping porous carbonpaper in a dispersion of about 60 weight percent polytetrafluoroethyleneso that the carbon paper has a loading of about 20 mg./cm. ofpolytetrafluoroethylene; providing an alloy catalyst consistingessentially of platinum alloyed with at least one metal selected fromthe group consisting of ruthenium, rhodium, iridium and nickel, saidplatinum comprising 50 to 80 weight percent of the alloy;

depositing the alloy catalyst on a finely divided support, said alloycatalyst comprising 10 to 35 weight percent of the supported alloycatalyst;

mixing the supportedalloy catalyst with a hydrophobic polymer, saidsupported alloy catalyst comprising 70 to 90 weight percent of themixture;

providing a wetting agent/water solution, said wetting agent comprising0.2 to 5.0 volume percent of the solution;

suspending the mixture of supported alloy catalyst and hydrophobicpolymer in the wetting agent/water solution in the proportion of 1 gm.of alloy catalyst to 4 to milliliters of solution;

applying said suspension onto said substrate so that the alloy loadingis no greater than 1 mg./cm. drying the coated substrate to remove waterthen pressing the coated substrate;

further drying the coated, pressed substrate until the wetting agent isremoved; and then sintering the coated, pressed dried substrate. 2. Amethod of fabricating a lightweight fuel cell electrode having a noblemetal content no greater than i mg./cm., comprising the steps of:

preparing an electrically conductive substrate by pressing a thin filmof material selected from the group consisting ofpolytetrafluoroethylene, aluminum foil and waxed paper into a screen tofill about one-half the void volume of the screen; providing an alloycatalyst consisting'essentially of platinum alloyed with at least onemetal selected from the group consisting of ruthenium, rhodium, iridiumand nickel, said platinum comprising 50 to 80 weight percent of thealloy;

depositing the alloy catalyst on a finely divided support, said alloycatalyst comprising 10 to 35 weight percent of the supported alloycatalyst;

mixing the supported catalyst with a hydrophobic polymer, said supportedalloy catalyst comprisingto weight percent of the mixture;

providing a wetting agent/water solution, said wetting agent comprising0.2 to 5.0 volume percent of the solution;

suspending the mixture of supported alloy catalyst and hydrophobicpolymer in the wetting agent/water solution in the proportion of 1 gm.of alloy catalyst to 4 to 20 milliliters of solution;

applying said suspension onto said substrate so that the alloy loadingis no greater than 1 mg./cm. drying the coated substrate to remove waterthen pressing the coated substrate;

further drying the coated, pressed substrate until the wetting agent isremoved; and then sintering the coated, pressed dried substrate.

3. A method of fabricating a lightweight electrode as defined in claim2, wherein the thin film is removed prior to sintering.

4. A method of fabricating a lightweight electrode as defined in claim2, wherein the support is selected from the group consisting ofgraphite, carbon and boron carbide.

5. A method of fabricating a lightweight electrode as defined in claim2, wherein the further drying step is carried out between 250 and 350]?6. A method of fabricating a lightweight electrode as defined in claim2, wherein the sintering step is carried out between 570 and 620F.

7. A method of fabricating a lightweight electrode, as defined in claim2, wherein the metal alloyed with platinum is ruthenium.

2. A method of fabricating a lightweight fuel cell electrode having anoble metal content no greater than 1 mg./cm.2, comprising the steps of:preparing an electrically conductive substrate by pressing a thin filmof material selected from the group consisting ofpolytetrafluoroethylene, aluminum foil and waxed paper into a screen tofill about one-half the void volume of the screen; providing an alloycatalyst consisting essentially of platinum alloyed with at least onemetal selected from the group consisting of ruThenium, rhodium, iridiumand nickel, said platinum comprising 50 to 80 weight percent of thealloy; depositing the alloy catalyst on a finely divided support, saidalloy catalyst comprising 10 to 35 weight percent of the supported alloycatalyst; mixing the supported catalyst with a hydrophobic polymer, saidsupported alloy catalyst comprising 70 to 90 weight percent of themixture; providing a wetting agent/water solution, said wetting agentcomprising 0.2 to 5.0 volume percent of the solution; suspending themixture of supported alloy catalyst and hydrophobic polymer in thewetting agent/water solution in the proportion of 1 gm. of alloycatalyst to 4 to 20 milliliters of solution; applying said suspensiononto said substrate so that the alloy loading is no greater than 1mg./cm.2; drying the coated substrate to remove water then pressing thecoated substrate; further drying the coated, pressed substrate until thewetting agent is removed; and then sintering the coated, pressed driedsubstrate.
 3. A method of fabricating a lightweight electrode as definedin claim 2, wherein the thin film is removed prior to sintering.
 4. Amethod of fabricating a lightweight electrode as defined in claim 2,wherein the support is selected from the group consisting of graphite,carbon and boron carbide.
 5. A method of fabricating a lightweightelectrode as defined in claim 2, wherein the further drying step iscarried out between 250* and 350*F.
 6. A method of fabricating alightweight electrode as defined in claim 2, wherein the sintering stepis carried out between 570* and 620*F.
 7. A method of fabricating alightweight electrode, as defined in claim 2, wherein the metal alloyedwith platinum is ruthenium.